High power pulsed microwave source

ABSTRACT

Microwave pulses at power levels in excess of 10 megawatts and of short duration and narrow bandwidth are produced by propagating an annular relativistic electron beam carrying currents of 30-40 kA at energies of 200-500 keV through a cylindrical, symmetrical annular, ridged wave guide. The interaction between this beam and a backward wave in the guide leads to the oscillations.

United States Patent Nation Oct. 24, 1972 [541 HIGH POWER PULSED MICROWAVE SOURCE Inventor: John A. Nation, Ithica, NY.

The United States of America as represented by the Secretary of the Navy Filed: Aug. 30, 1971 Appl. No.: 175,887

Assignee:

us. Cl. ..31s/3.s, 313/84, 315/537 rm. Cl ..H0lj 25/34 Field of Search ..31s/3.s, 39.3, 5.39, 3.6;

References Cited UNITED STATES PATENTS l/1970 Denholm et al. ..315/5.39 4/1970 Poschl et a1..' ..315/3.5 X

NEGATIVE PULSE FORMING CIRCUIT 3,054,018 9/1962 Paschke ..315/3.6 2,632,120 3/1953 Hull ..313/84 X 3,315,110 4/1967 Wang ..3l3/84 3,489,943 l/ 1970 Denholm ..3l3/336 X 3,076,115 l/l963 Smith ..315/3.5 3,273,011 9/1966 Brown ..3l5/3.5 X

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attorney-R. S. Sciascia et al.

[57] ABSTRACT Microwave pulses at power levels in excess of 10 megawatts and of short duration and narrow bandwidth are produced by propagating an annular relativistic electron beam carrying currents of 30-40 kA at energies of 200-500 keV through a cylindrical. symmetrical annular, ridged wave guide. The interaction between this beam and a backward wave in the guide leads to the oscillations.

PATENTEBUBI 24 1912 NEGATIVE PULSE FORMING SWITCH CIRCUIT v% "'l'r/q'r/q. J

in] A f 7.0GHz

0.5 l I I l l t FREQUENCY (GHz) Fig. 2

HIGH POWER PULSED MICROWAVE SOURCE The present invention relates generally to highpower, pulsed microwave sources and, more particularly, to apparatus for and methods of producing microwave pulses at power levels in excess of 100 megawatts.

There are available in the prior art, arrangements for generating high intensity pulsed microwave signals. However, those systems which employ a bunched electron beam require complicated control circuits for accomplishing the necessary pulse modulation. This modulation becomes all the more difficult, if indeed possible, where beams in the order of 10,000 amps are involved. Such power levels are desirable in planetary radar, civil and military radar systems, microwave heating of gases and plasmas and ion accelerators.

It is accordingly a primary object of the present invention to generate short duration, high power, microwave pulses through the coupling of a high current beam of relativistic electrons to a slow wave structure.

Another object of the present invention is to provide microwave pulses at power levels in excess of megawatts and of short duration and narrow bandwidth.

Another object of the present invention is to provide a microwave generating apparatus wherein electron beam generation and modulation are achieved simultaneously.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 schematically illustrates one modification of the present invention wherein a field emission diode is utilized as the means for generating the relativistic beam of electrons; and

FIG. 2 is a plot of the calculated frequency of the microwave energy as a function of the electron injection velocity and a comparison of this calculation with experiments.

Referring now to FlG..l of the drawings, the pulsed microwave source has as its electron beam injecting gun a solid metallic cathode 1 in the shape of a blunted cone. Appropriate circumferential areas of its slant surface are roughened to enhance electron emission therefrom. This same effect may be achieved with any dielectric inclusion in the slant surface. The cathode material is also nonmagnetic.

Surrounding the cathode is an anode structure 2 of frustro-conical shape whose slant surface parallels that of the cathode to define a channel area 3. This anode is made of thin, metallic material so as to permit the pulse magnetic field which controls the deflection and focusing of the electrons to readily penetrate it with little perturbation. The pulse magnetic field is derived from a magnetic coil 4 disposed about the anode structure and energized from a current pulse generator 5.

Cathode I and anode 2 constitute a field emission diode, and the disposition of the cathode within the magnetic field coil 4 and the electrical properties of this electrode are such that when a current pulse of suitable amplitude is supplied to the coil, the magnetic lines of force during the buildup to maximum field strength are parallel to the slant surfaces of the cathode and anode in the channel area 3. One of the reasons this result obtains is that the nonmagnetic properties and the solid construction of the cathode prevent any appreciable magnetic penetration until the peak magnetic period. As an alternative to the above construction, the electron beam injection apparatus may utilize a continuous magnetic field so as to relax the condition for a thin anode wall structure. However, this arrangement would require specially designed field coils to give the correct field profile.

When the magnetic field has reached its peak intensity, a negative voltage pulse of an amplitude, for example, in the hundreds of kilovolts range, is applied to the cathode. With the anode grounded, electron emission takes place from the roughened areas of the cathode by the cold field emission process. The emitted electrons, instead of proceeding to the plate electrode, are diverted therefrom by the magnetic field, and this field directs them down through the channel towards an evacuated drift space 6.

The negative voltage pulses may be derived from a Blumlein or other transmission line system 7 so as to possess the short voltage and current rise times and duration necessary, in the present invention, to both initiate the electron beam and also modulate it at the same time.

It should be appreciated that instead of driving the cathode highly negative with respect to the anode by means of the negative voltage pulse mentioned hereinbefore, the cathode itself may be grounded and the anode pulsed to a high positive level.

The annular beam of bunched electrons entering the drift space 6 coacts with a slow wave structure 8 which, in the present case, is in the form of a cylindrical conducting element having its outer surface circumferentially notched at spaced longitudinal locations so as to define, with a surrounding conductor 9 which is at the anode potential, a ridged wave guide structure. In order to preserve the focusing of the annular beam, additional coils, such as 10, may be disposed along the drift section to produce a suitable axial magnetic field in the order of a few kiloGauss.

The energy associated with the moving electron beam couples to the radio frequency fields of the ridged wave guide structure in a manner similar to that found in traveling wave-type tubes, and the resultant microwave signal is radiated in a beam from the remote end of the apparatus or coupled from the guide by other means. The onset of this radiation, in one particular embodiment, was observed within 5 nanoseconds from the time the initiating voltage pulse was applied to the field emission diode structure.

FIG. 2 illustrates the calculated plot and experimental data of the normalized electron injection velocity B v/c versus oscillation frequency in gigahertz, where the velocity of the electrons was computed on the basis of applied voltage pulse amplitude. The two curves represent the azimuthally symmetric backward waves for two different background plasma densities. It will be seen that the radiated frequency is primarily dependent on the ejection velocity of the beam and independent of the magnetic field. This result, together with the comparison with the calculated curve, suggests that the beam interaction occurred primarily with the backward wave. With proper design, it is also possible to couple to a forward wave.

The voltage pulse was varied from 700 kV down to 220 kV. As the voltage was reduced, the frequency of the radiated signal was observed to vary from 9.7 GHz to 7.8 GHz. Careful analysis of the microwave pulse showed that the bandwidth was approximately 50 MHz and depended primarily on the injection velocity of the electrons. In this modification the pulse duration was 50 nanoseconds and the microwave emission of 30 nanoseconds duration. In the modification mentioned hereinbefore, the radiated power was estimated to be in excess of megawatts and was propagated in a diverging beam of half angle of approximately What is claimed is:

1. Apparatus for generating microwave energy pulses in the megawatt power range comprising, in combination,

a hollow anode electrode of frustro-conical shape;

a cathode electrode of conical shape,

said cathode being centrally positioned within the interior of said hollow anode so that an annular channel area exists between the outer surface of said cathode and the inner surface of said anode; means for generating a pulsed magnetic field within said channel area whose direction is parallel to the slant surface of said cathode; means for applying to one of said electrodes a short duration voltage pulse when said pulsed magnetic field has maximum intensity, the rise time, length and magnitude of said voltage pulse being such that electrons emanate from said cathode by a cold field emission process and form a pulsed annular beam of relativistic electrons carrying currents of kA magnitude;

cylindrical conducting member having its outer surface notched over its length; and

hollow, cylindrical conductor surrounding said cylindrical member and spaced therefrom such that said notched surface and the inner wall surface of said conductor form a ridged circular wave guide section, said cylindrical member and said cylindrical conductor being positioned such that said pulsed annular electron beam enters one end of said ridged circular wave guide section and thereafter travels down the length thereof,

said electron beam and the backward wave produced therefrom which is propagating in an opposite direction to said beam interacting to generate a megawatt microwave energy pulse which is radiated from the other end of said ridged wave guide section.

- 2. In arrangement as defined in claim 8, means for establishing an axial magnetic field through said wave guide section so as to maintain said beam in focus.

3. In an arrangement as defined in claim 1, wherein said cathode is made of nonmagnetic, metallic material and is of solid design.

4. In an arrangement as defined in claim 3, wherein a circumferential area of said cathode surface is treated so as to enhance electron emission therefrom.

5. In an arrangement as defined in claim 1, wherein said anode is made of thin material which allows a magnetic field to penetrate therethrough without significant distortion. 

1. Apparatus for generating microwave energy pulses in the megawatt power range comprising, in combination, a hollow anode electrode of frustro-conical shape; a cathode electrode of conical shape, said cathode being centrally positioned within the interior of said hollow anode so that an annular channel area exists between the outer surface of said cathode and the inner surface of said anode; means for generating a pulsed magnetic field within said channel area whose direction is parallel to the slant surface of said cathode; means for applying to one of said electrodes a short duration voltage pulse when said pulsed magnetic field has maximum intensity, the rise time, length and magnitude of said voltage pulse being such that electrons emanate from said cathode by a cold field emission process and form a pulsed annular beam of relativistic electrons carrying currents of kA magnitude; a cylindrical conducting member having its outer surface notched over its length; and a hollow, cylindrical conductor surrounding said cylindrical member and spaced therefrom such that said notched surface and the inner wall surface of said conductor form a ridged circular wave guide section, said cylindrical member and said cylindrical conductor being positioned such that said pulsed annular electron beam enters one end of said ridged circular wave guide section and thereafter travels down the length thereof, said electron beam and the backward wave produced therefrom which is propagating in an opposite direction to said beam interacting to generate a megawatt microwave energy pulse which is radiated from the other end of said ridged wave guide section.
 2. In arrangement as defined in claim 8, means for establishing an axial magnetic field through said wave guide section so as to maintain said beam in focus.
 3. In an arrangement as defined in claim 1, wherein said cathode is made of nonmagnetic, metallic material and is of solid design.
 4. In an arrangement as defined in claim 3, wherein a circumferential area of said cathode surface is treated so as to enhance electron emission therefrom.
 5. In an arrangement as defined in claim 1, wherein said anode is made of thin material which allows a magnetic field to penetrate therethrough without significant distortion. 